1. Field of the Invention
The present invention concerns electromagnetic actuators producing a linear motion, and more particularly concerns electromagnetic actuators serving as prime movers to produce bi-directional, pushing and pulling, motion and force.
2. Background Art
The electromagnetic actuator in accordance with the present invention will be seen to serve as a prime mover producing, by consumption of electrical energy, linear motion and force between two stable positions where no electrical energy is consumed. The motions undergone, and the forces produced, by the actuator of the present invention are similar to those motions and forces previously derived from solenoids, particularly solenoids of the two-position self-holding type.
A solenoid is intrinsically a device which operates under electrical energization of a coil to pull a solenoid plunger into a position that provides the magnetic field generated by the coil with a magnetic path of minimum reluctance. A pushing movement may be realized from the normal pulling action of a solenoid by use of a lever, or by use of a return spring which is overcome by a solenoid of sufficient force capability. Alternatively, a non-magnetic extension to a solenoid plunger may protrude through a surrounding coil and through the end polepiece and case of the solenoid in the direction of the plunger's movement. When such a non-magnetic plunger extension is present, it interferes with the normal path of magnetic flux, and reduces the efficiency of the solenoid.
Thus the common implementation of a two-position solenoid is simply two back-to-back solenoids. A switch energizes either one solenoid coil, or the other, in order to achieve a pushing, or a pulling, motion. If the two position solenoid is also self-holding, meaning that it need not consume electrical power in order to stably maintain each of its two positions, then it must additionally incorporate some mechanism that holds the solenoid plunger at its alternate positions. Such function can be accomplished by use of mechanical "over-center" devices, such as a Belville disk, or by use of permanent magnets to hold the prime mover in position. Note that in all such latching schemes, wherein the latching device is not inherent in the prime mover, the latching forces realized must always be substantially less than the solenoid force required to overcome the latching mechanism. Thus, the useful output forces of the whole device are less than can be achieved without latching mechanisms.
One preferred embodiment of an electromagnetic actuator in accordance with the present invention will be seen to be micropowered and to achieve a self-holding without any loss of output force. A comparable previous mechanism is the two-position self-holding solenoid part no. SH2L-0224 (NP-15) available from Electro-Mechanisms, Inc., P.O. Box A, Azuza, Calif. 91702. This miniature solenoid, from a manufacturer that specializes in such devices, has a single plunger that moves, responsively to energization of a selected one of two separate coils, in each of two directions. After movement to one end of its path the solenoid plunger is thereafter held in position by a permanent magnet that is affixed to the plunger, and that magnetically contracts the housing of that solenoid coil to which it becomes most closely positioned. Because of this attraction, the solenoid's plunger is held in position even in the absence of any applied holding current.
The electromagnetic actuator in accordance with the present invention will be seen to be highly efficient in the consumption of electrical energy. It is thus illustrative to calculate the energy efficiency of a previous two-position solenoid device, for example the aforementioned SH2L-90224 (NP-15) solenoid device. The moving force of the solenoid plunger has been characterized, together with the strength of the electrical magnetization of the solenoid coil. For a nominal energization of 2.8 volts for a time duration of 5 milliseconds the solenoid plunger of the Electro-Mechanisms, Inc. device will traverse a path of 0.8 mm developing a maximum force of 20 grams. This force will be seen to be roughly equivalent to that force that will be seen to be developed by the preferred embodiment of an electromagnetic actuator in accordance with the present invention. Therefore the energy efficiencies in producing this force in the previous device of Electro-Mechanisms, Inc. (as typical of the solenoid art), and in the device in accordance with the present invention, may be useful compared.
An energy efficiency factor for an electromagnetic actuator may be defined as the work output divided by the energy input. In MKS units, this efficiency will equal Newtons force output times meters of stroke divided by joules (watt seconds) times 100%, and will be expressed in newtons times meters divided by joules (N.multidot.M/J) times 100%--a dimensionless quotient.
For the Electro-Mechanisms, Inc. two-position self-holding solenoid type SH2L-0224 the coil resistance is 4.3 ohms. The energy may thusly be calculated as follows: ##EQU1## The stroke of the solenoid is 0.8 millimeters. The work may thusly be calculated as follows: ##EQU2## The force F(x) is not constant over the length of solenoid plunger travel between points 1 and 2, but may conservatively be estimated to be less than or equal to 20 grams over the entire distance of travel. Therefore, as a simplication: ##EQU3## The arbitrarily-defined energy efficiency of this particular previous electrical solenoid, as representative of the solenoid art, is calculated as follows: ##EQU4##
The energy efficiency of a particular preferred embodiment of an electromagnetic actuator in accordance with the present invention will be seen to be approximately ten times (.times.10) better than this calculated figure. (The efficiency of this particular preferred embodiment will be seen to be reduced from optimal efficiency because the electromagnetic sections of the actuator will be seen to be isolated by a plastic barrier from fluid water, the flow of which is gated in an exemplary application of the actuator to power a valve. When electromagnetic actuators in accordance with the invention are employed as prime movers in a dry environment their efficiency is anticipated to be roughly two orders of magnitude better than this calculated figure.) Moreover, the actuator in accordance with the present invention will both push and pull by selective electrical energization of a single coil.
The switching of the flux of a permanent magnet by use of an electromagnet is also relevant to the present invention. A previous device that employs flux switching, although not in the manner of the present invention, is the Magnelatch option for the solenoid valves of Skinner Electric Valve Division, New Britain, Conn. The magnelatch option, described as unique in solenoid valve operation, employs a permanent magnet latch circuit for a solenoid valve. Current to maintain the valve in either one of its two positions is not required, as will be seen to also be the case with the actuator in accordance with the present invention. The magnelatch option valve of Skinner Electric Valve includes (1) a saddle, or flux, plate; (2a) a main, or latch, coil, (2b) a switch coil, (3a) a large permanent magnet PM1 used to latch a plunger, (3b) a small permanent magnet PM2 the polarity of which can be switched to properly function the valve, (4) pole pieces serving as positioners for magnetic switch PM2, (5) a saddle coupling to encase PM1 and ensure its proper placement in a flux circuit, and (6) a sole, or lower flux, plate.
In operation, a Magnelatch option solenoid valve switches the flux of a small permanent magnet, PM2 by use of a dedicated switch coil. The magnetic flux generated by PM1 may be either in phase with, or out of phase with, a much stronger permanent magnetic flux generated by PM2. The plunger magnetic circuit is surrounded by a gap which is non-magnetic and which provides a high reluctance path. Following the path of least reluctance, the combined flux of PM1 and PM2 will pass along two different circuits dependent upon the current magnetization of switch magnet PM1. In one such circuit, the combined flux of PM1 and PM2 will pass through an outer circuit consisting of PM1, the saddle plate, the PM2 poles, PM2 itself, and the sole plate. In this condition the magnetic circuit has no effect on the plunger, and a spring force and/or fluid pressure is used to hold the plunger on a seat of the valve.
When a momentary pulse of direct current, having correct polarity and duration of approximately 20 milliseconds, is provided to the dedicated coil assembly of switch magnet PM2, it causes PM2 to switch its polarity and to thereafter repel the flux generated by PM1. This action causes the full flux output of PM1 to shunt across the plunger magnetic circuit because this inner circuit now has a lower reluctance than the outer circuit. When the flux travels through the plunger circuit it causes the plunger to move up against a stop and to open an orifice, permitting fluid flow through the valve.
The relevance of the Magnelatch option to the present invention is primarily for showing that the flux of a permanent magnet may be switched, and, if it is so switched, that it can provide forces of useful magnitude in the operation of a solenoid-type device.
In still another area, it is known to use solenoids to actuate hydraulic valves of the diaphragm type. In such valves water from a supply line enters the valve inlet and pressurizes a seat area. This forces a diaphragm away from the seat and the valve opens. A solenoid is selectively actuated to flow the pressurized water through a control conduit to a chamber on the opposite side of the solenoid from the seat area. The area of the diaphragm in the chamber is larger than the valve seat area, producing a net force on the diaphragm toward the valve seat and closing the valve.
Such a hydraulic valve is "normally open", and requires solenoid actuation to close. Hydraulic valves may alternatively be constructed to be "normally closed".
A particular configuration of a diaphragm valve called a 3-way solenoid diaphragm valve is of relevance to one preferred application of an electromagnetic actuator in accordance with the present invention. One such 3-way solenoid diaphragm valve is a Buckner.RTM. valve (registered trademark of Buckner, Inc. 4381 N. Brawley Avenue, Fresno, Calif. 93722). Such Buckner.RTM. 3-way solenoid diaphragm valve uses a three-way solenoid that controls three orifices to the valve: two orifices to a control chamber and a major orifice through which movement of a diaphragm permits fluid to flow. There is no water path through the center of the diaphragm. Water from a supply line enters the chamber above the diaphragm through an inlet port under solenoid control. Because the area on top of the diaphragm is larger than area below the diaphragm at the valve seat, pressure is greater above diaphragm and the valve closes.
When the solenoid is energized the inlet port is closed and simultaneously a vent port opens at the top of the solenoid. Water from the chamber above the diaphragm is vented to atmosphere through the vent port, lowering the pressure above the diaphragm. Since the pressure is now greater under the diaphragm at the valve seat, valve opens and remains open as long as solenoid is energized and the inlet port is closed.
Notably to the present invention, water flows through the electrical sections of the solenoid in the Buckner.RTM. 3-way solenoid diaphragm valve. The necessity of making these sections waterproof increases costs, reduces electrical efficiency due to the increased mechanical separation between magnetic elements in order to accommodate waterproof barriers, and hazards failure if water shorts the electrical circuit. A preferred application of an electromagnetic actuator in accordance with the present invention will be seen to perform the selective occluding of two orifices to a control chamber of a 3-way diaphragm valve totally without contact between the gated water and the electrical sections of the actuator, or without significant hazard that such contact will occur.